    //文档高度
  export function getDocumentTop() {
      var scrollTop = 0, bodyScrollTop = 0, documentScrollTop = 0;
      if (document.body) {
          bodyScrollTop = document.body.scrollTop;
      }
      if (document.documentElement) {
          documentScrollTop = document.documentElement.scrollTop;
      }
      scrollTop = (bodyScrollTop - documentScrollTop > 0) ? bodyScrollTop : documentScrollTop;
      return scrollTop;
  }
  
  //可视窗口高度
  export function getWindowHeight() {
      var windowHeight = 0;
      if (document.compatMode == "CSS1Compat") {
          windowHeight = document.documentElement.clientHeight;
      } else {
          windowHeight = document.body.clientHeight;
      }
      return windowHeight;
  }
  
  //滚动条滚动高度
  export function getScrollHeight() {
      var scrollHeight = 0, bodyScrollHeight = 0, documentScrollHeight = 0;
      if (document.body) {
          bodyScrollHeight = document.body.scrollHeight;
      }
  
      if (document.documentElement) {
          documentScrollHeight = document.documentElement.scrollHeight;
      }
      scrollHeight = (bodyScrollHeight - documentScrollHeight > 0) ? bodyScrollHeight : documentScrollHeight;
      return scrollHeight;
  }
  


